1. Field of the Invention
The present invention relates to a flip-chip mounted surface acoustic wave (SAW) device and a method of producing the same.
2. Description of Related Art
Conventionally, surface-acoustic-wave (SAW) devices are fabricated through a series of steps of adhesively fixing a SAW chip on the inside bottom surface of a package and then interconnecting the bonding pads formed in the package and the bonding pads on the surface of the SAW chip through the wire bonding. However, these wire-bonded-type SAW devices have a problem in that the adhesive agent used for fixing the SAW chip leads to variations in characteristic after fabrication. In contrast, flip-chip mounted-type SAW devices are fabricated through the following steps. That is, bumps made of a conductor such as gold are placed on conductive pads of a SAW chip. The SAW chip is placed on the bottom surface inside the package, with the bump formed surface (electrode formed surface) facing the bottom surface inside the package (die attachment surface). The intermediate structure is subjected to heating, pressure, or ultrasonics. Thus, the conductive pads on the die attachment surface and those on the electrode formed surface are electrically connected and mechanically fixed by the bumps. This process does not require any adhesive agent. Hence, the resultant SAW device has less variation in characteristics than do conventional wire-bonded-type SAW devices.
FIG. 7 illustrates the configuration of a flip-chip mounted SAW device. Referring to FIG. 7, electrodes or various conductive layers (not shown) are formed on the electrode formed surface 10a of a SAW chip 10. The electrodes may be ones that excite a surface of a Piezoelectric substrate of, e.g. quartz crystal, by applied electrical signals to generate surface acoustic waves. The electrodes may receive surface acoustic waves and produce electrical signals or may reflect surface acoustic waves in a specific direction. Moreover, conductive pads (not shown) electrically connected with the electrodes on the electrode formed surface 10a are connected to and fixed to the package 12. The package 12 has a recess to accommodate the SAW chip 10. Conductive pads (not shown) are formed on the bottom surface, or the die attachment surface 12a, so as to face bumps 14 when the chip 10 is accommodated in the recess. Footprints 12b, being conductors for external connection, are formed on the package 12. The footprints 12b are electrically connected to conductive pads on the die attachment surface 12b through conductors (not shown) penetrating the package 12.
In the fabrication of the SAW device in FIG. 7, bumps 14 made of a metal such as gold are first placed on the conductive pads formed on the electrode formed surface 10a. The SAW chip 10 is placed in the recess of the package 12, with the electrode formed surface 10a downward in FIG. 7. The intermediate structure is subjected to heating, pressure, or ultrasonics. Thus, the conductive pads of the SAW chip 10 are electrically connected and mechanically fixed to the conductive pads on the die attachment surface 12a via the bumps 14. Thereafter, the lid (cover) 12c seals the recess to house the SAW chip 10. The sealing portion 12d, which is a sealing member intervening between the lid 12c and the package 12, is formed by applying heat or pressure. Gold or tin, resin, soldering, glass and so on are among the substances that may be used for the sealing member.
With the package 12 connected and fixed with the bumps 14, the characteristics of the SAW device 10 can be measured with an external measuring device via the footprints 12b before sealing the opening of the package 12. One of the inventors of the present invention also invented the method disclosed in Japanese Patent Laid-Open Publication No. Hei NO. 10-145167 (hereinafter referred to as a previous proposal). According to this method, before the opening of the package 12 is covered with the lid 12c after the bump connection and fixture, focused radiation such as laser light is irradiated from above the back surface of the SAW chip 10 (from the upper orientation in FIG. 7), to adjust the characteristics of the SAW chip 10. Thus, the radiation evaporates the conductive layer or the insulating layer on the electrode formed surface 10a while the characteristics of the in-process item is being measured. Though this process, the device characteristics can be adjusted to suppress and prevent characteristic variations due to conditional deviations in fabrication.
It is an object of the present invention to provide a flip-chip mounted surface-acoustic-wave device, the characteristics of which can be adjusted during fabrication to suppress changes in characteristics due to fabrication.
Another object of the present invention is to provide a surface-acoustic-wave device that adopts a devised characteristic adjusting method so that variation in characteristics can be suppressed to a larger degree, compared with the previous proposal.
In order to achieve the above mentioned objectives, a method of fabricating a surface-acoustic-wave device according to the present invention comprises (1) a mounting step of flip-chip mounting a surface-acoustic-wave (SAW) chip within a package, the package having a window for characteristics adjustment in its die attachment surface, and (2) a first adjustment step of adjusting characteristics of an in-process item by an adjustment operation. The adjustment operation may be the introduction of a gas into the package through the window, or may be the beam irradiation onto the electrodes or insulation films on the surface of the SAW chip, or may be the combination thereof.
The gas is introduced, for example, into the package to etch the substrate or electrodes of a SAW chip, or, onto the electrode formed surface of a SAW chip to form (e.g. by CVD) a film relating to surface acoustic wave propagation, or, onto the electrode or insulation film formed on the surface of the SAW chip to change or adjust the quality of the electrode or insulation film. Gas introduction in the preferable embodiment of the present invention can be defined as the introduction of gases which affect the surface-acoustic-wave propagation and thus the characteristics of a SAW device. That is, according to the present invention, since the substrate conditions and the electrode properties of a SAW chip can be changed, characteristics variations such as center frequency variations can be further suppressed and the center frequency can precisely be set to the target frequency. The gas to be introduced may be selected according to whether the substrate is to be etched or not, whether the electrode is to be etched or not, whether the quality of the electrodes or insulation films or not, whether a new film is to be formed, and so on.
According to the present invention, a window for characteristics adjustment is formed on the die attachment surface. Because the die attachment surface must be provided with an area for connecting a SAW chip to the package side, such as a conductive pad area, the window must be smaller than the opening for introducing a SAW chip into the package. The present invention may preferably embodied by effectively utilizing the relationship between the opening area and the window area.
In an example employing a method with a relatively high etching rate, like reactive ion etching wherein a fluorine series gas is used as an etching gas, a case can be considered wherein the substrate or electrodes of a SAW chip is etched to perform the device characteristics adjustment related to the feature of the present invention and wherein the in-process item to be adjusted in characteristics is a SAW device in process for a relatively high frequency band, that is, with fine electrodes. If plasma as used in this specification, comprehensively including excited active species such as ions, electrons, atoms, and radicals in this application) is introduced onto the electrode formed surface of a SAW chip through an opening area as large as the opening for chip accommodation, the etching rapidly advances. Hence, it may be difficult to set the characteristic of an in-process item, e.g. the center frequency of a pass band, to a target value, control is difficult.
According to the present invention, a window with an opening area smaller than that of the opening for chip accommodation is used to introduce gas for characteristics adjustment. Hence, when the substrate or electrodes of a SAW chip are etched using a method with a relatively high etching rate, plasma introduced onto the electrode formed surface of the SAW chip, and thus the etching rate can be suppressed. The present invention can prevent the control difficulty due to the high etching rate in the reactive ion etching by suppressing the etching rate and can accurately and easily adjust the characteristics of SAW devices designed for use in relatively high frequencies. This effect can be further improved by forming the window for characteristics adjustment as a set of plural openings, that is, by making each window smaller by dividing it into plural openings.
The document, xe2x80x9cFrequency Trimming of SAW Devicesxe2x80x9d, Takehiko Uno, Kanagawa Institute of Technology, 1994, IEEE Ultrasonic Symposium Proc. Vol. 1, pp. 181-187, Nov. 1-4, 1994, discloses a dry etching technique for adjusting characteristics of SAW devices. In this document, it should be noted that the SAW device is fabricated through the wire bonding, that the electrode formed surface of a SAW chip confronts the opening for chip accommodation, and that the etching gas is introduced through the opening for chip accommodation. Therefore, adjustment of the device characteristics using the technique disclosed in that document is difficult.
In the present invention, however, a first sealing step of sealing a window for characteristics adjustment with a lid for characteristics adjustment window is executed after a first adjustment step while a second sealing step of sealing an opening (used in an assembling step) for introducing a SAW chip in a package with a lid for accommodation opening is executed after at least the assembling step is executed. The fabrication process can be additionally simplified by concurrently performing the second sealing step and the first sealing step. When doing so, the sealing member to seal the opening with a lid for accommodation opening and the sealing member to seal the window with a lid for characteristic adjustment window may be made of the same material. The lid for characteristics adjustment window may partially protrude toward the package. However, such a problem can be avoided by disposing legs for electrical connection and mechanical fixing to external circuit, on the side of the window, and by setting the thickness of the lid for characteristics adjustment to smaller than the height of the legs. In this aspect, the lid for the characteristics adjustment window can be mounted without adversely affecting the external connection and fixture, while a small package size is preserved.
In addition to an introduction of gas through the window, light, such as laser light, or other electromagnetic radiation may be irradiated onto the selected portion of the electrode formed surface of a SAW chip via the window, so that the film qualities of electrodes or the adjacent insulating layers are changed to adjust characteristics of an in-process item. This step, or the second adjustment step, may be executed before the window is covered with a lid. Preferably, the second adjustment step may be performed together with the first adjustment step. The lid for characteristics adjustment window may be made of a substance transparent to light or electromagnetic radiation, then the second adjustment step may be executed after the first sealing step, making it possible to compensate for changes in characteristics, even when characteristics changes occur in the first sealing step. When the measurement step of measuring characteristics of an in-process item is executed after at least the assembling step, the characteristics adjustment amount of an in-process item in the first and/or second adjustment steps can be determined.
Moreover, the present invention may be defined as the invention of a SAW device. The SAW device of the present invention includes a package having a die attachment surface provided inside the package and a SAW chip flip-chip mounted on the die attachment surface in the package. A window for characteristics adjustment is formed in the wall with the die attachment surface. The window is closed with a lid for characteristics adjustment window. Before the window is sealed with the lid, gas is introduced via the window to adjust the characteristics of the SAW chip. Because of small variations in characteristics, the SAW devices according to the present invention can be used as resonators, filters, delay lines, or others of higher quality than with conventional SAW devices.